feat: render dragging links above Vue nodes via overlay canvas (#8695)

## Summary
When vueNodesMode is enabled, the dragging link preview was rendered on
the background canvas behind DOM-based Vue nodes, making it invisible
when overlapping node bodies.

Add a new overlay canvas layer between TransformPane and
SelectionRectangle that renders the dragging link preview and snap
highlight above the Vue node DOM layer.

Static connections remain on the background canvas as before.

fix https://github.com/Comfy-Org/ComfyUI_frontend/issues/8414
discussed with @DrJKL 

## Screenshots
before

https://github.com/user-attachments/assets/94508efa-570c-4e32-a373-360b72625fdd

after

https://github.com/user-attachments/assets/4b0f924c-66ce-4f49-97d7-51e6e923a1b9

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8695-feat-render-dragging-links-above-Vue-nodes-via-overlay-canvas-2ff6d73d365081599b2fe18b87f34b7a)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

src/components/graph/GraphCanvas.vue

@@ -76,6 +76,13 @@
     />
   </TransformPane>
 
+  <LinkOverlayCanvas
+    v-if="shouldRenderVueNodes && comfyApp.canvas && comfyAppReady"
+    :canvas="comfyApp.canvas"
+    @ready="onLinkOverlayReady"
+    @dispose="onLinkOverlayDispose"
+  />
+
   <!-- Selection rectangle overlay - rendered in DOM layer to appear above DOM widgets -->
   <SelectionRectangle v-if="comfyAppReady" />
 
@@ -111,6 +118,7 @@ import BottomPanel from '@/components/bottomPanel/BottomPanel.vue'
 import ExtensionSlot from '@/components/common/ExtensionSlot.vue'
 import DomWidgets from '@/components/graph/DomWidgets.vue'
 import GraphCanvasMenu from '@/components/graph/GraphCanvasMenu.vue'
+import LinkOverlayCanvas from '@/components/graph/LinkOverlayCanvas.vue'
 import NodeTooltip from '@/components/graph/NodeTooltip.vue'
 import SelectionToolbox from '@/components/graph/SelectionToolbox.vue'
 import TitleEditor from '@/components/graph/TitleEditor.vue'
@@ -241,6 +249,18 @@ const allNodes = computed((): VueNodeData[] =>
   Array.from(vueNodeLifecycle.nodeManager.value?.vueNodeData?.values() ?? [])
 )
 
+function onLinkOverlayReady(el: HTMLCanvasElement) {
+  if (!canvasStore.canvas) return
+  canvasStore.canvas.overlayCanvas = el
+  canvasStore.canvas.overlayCtx = el.getContext('2d')
+}
+
+function onLinkOverlayDispose() {
+  if (!canvasStore.canvas) return
+  canvasStore.canvas.overlayCanvas = null
+  canvasStore.canvas.overlayCtx = null
+}
+
 watchEffect(() => {
   LiteGraph.nodeOpacity = settingStore.get('Comfy.Node.Opacity')
 })

src/components/graph/LinkOverlayCanvas.vue

@@ -0,0 +1,43 @@
+<template>
+  <canvas
+    ref="canvasRef"
+    class="pointer-events-none absolute inset-0 size-full"
+  />
+</template>
+
+<script setup lang="ts">
+import type { LGraphCanvas } from '@/lib/litegraph/src/LGraphCanvas'
+
+import { useRafFn } from '@vueuse/core'
+import { onBeforeUnmount, onMounted, ref } from 'vue'
+
+const { canvas } = defineProps<{
+  canvas: LGraphCanvas
+}>()
+
+const emit = defineEmits<{
+  ready: [canvas: HTMLCanvasElement]
+  dispose: []
+}>()
+
+const canvasRef = ref<HTMLCanvasElement | null>(null)
+
+useRafFn(() => {
+  const el = canvasRef.value
+  const mainCanvas = canvas.canvas
+  if (!el || !mainCanvas) return
+
+  if (el.width !== mainCanvas.width || el.height !== mainCanvas.height) {
+    el.width = mainCanvas.width
+    el.height = mainCanvas.height
+  }
+})
+
+onMounted(() => {
+  if (canvasRef.value) emit('ready', canvasRef.value)
+})
+
+onBeforeUnmount(() => {
+  emit('dispose')
+})
+</script>